it is not possible to knock out another player.
This is a long debated topic, but it boils down to…
a game needs to be fun for all players, a system that could allow one player to take away another players control of their character would be abused.
while I realize you are a RP crowd and the action is probably consensual, this would not be the case across all platforms and playstyles.
So this is something FC has said they will not include.
There was talk about Mods to do this, but I have no idea if there are any.